As the number of electronic elements fabricated on and within a semiconductor with integrated circuits continues to rise, problems of reducing and eliminating defects in the elements become more difficult to solve. To increase semiconductor capacity, circuit designers have reduced the size of individual elements to maximize available space on the semiconductor. The reduced size makes the electronic elements increasingly susceptible to defects caused by material impurities during fabrication, however. These defects are identified upon completion of the integrated circuit by testing procedures either at the semiconductor chip level or after complete packaging. Scrapping or discarding an entire chip because of a finding of defective electronic elements is economically undesirable, particularly if only a small number of electronic elements are actually defective.
Relying upon zero defects in fabrication of integrated circuits is not a realistic option either. To reduce the amount of semiconductor scrap, therefore, redundant elements are fabricated on the chip. If a primary element is determined to be defective, a redundant element may be substituted for the defective element. Substantial reductions in semiconductor chip scrap can be achieved through the use of redundant elements.
One type of integrated circuit device which uses redundant elements is electronic memory. Typical memory circuits comprise millions of equivalent memory cells arranged in addressable rows and columns. By fabricating redundant elements, either as rows or columns, defective primary rows or columns are replaceable. Thus, using redundant elements reduces scrap without substantially increasing the cost of the memory circuit.
Fusible conductive links, i.e. fuses, are used in rewiring electrical circuits in order to replace defective elements with redundant elements. The circuits are rewired by rendering selected fuses non-conductive, i.e., blown, by applying energy such as laser energy to the fuse with a device such as a laser trimming machine.
In dynamic or static memory chips, defective memory cells are replaced by blowing the fuses associated with the defective cells and activating a spare row or column of redundant cells. This circuit rewiring uses fusible links and considerably enhances yields and reduces production costs.
Logic circuits may also be repaired or reconfigured by blowing fuses. For instance, it is common to initially fabricate a generic logic chip having a large number of interconnected logic gates. In a processing step, the chip is customized to perform a desired logic function by disconnecting the unnecessary logic elements by blowing the fuses that connect them to the desired circuitry.
Semiconductor chips include fusible link regions and protective insulating layers over the fusible link regions. Openings are defined through the protective insulating layers and over fusible link regions to allow a laser to irradiate the fuse. These fuse openings frequently lower chip yields and circuit reliability by allowing contamination to penetrate from the openings to the device regions.
A laser is one energy source that is typically used to blow fuses. The laser is focused through the fuse opening and irradiates the fuse. For circuits described in U.S. Pat. No. 5,729,041 ('041) which issued Mar. 17, 1998, the fabrication of the circuit includes a step of defining an opening in an area of the fuse where laser heating is most effective in breaking the fuse. Because passivation layers overlying the fuse reduce laser energy striking the fuse, the passivation layers are etched away so that the fuse is exposed or so that only a single, thin, insulating layer or a portion of an insulating layer covers the fuse.
The fuse absorbs heat from the laser irradiation and the fuse melts. In this operation, called laser trimming, a rapid temperature rise of an upper portion of the fuse causes an increase in pressure within the circuit region. The pressure causes any overlying film to be blown off. A melted polysilicon fuse is removed from the semiconductor device by evaporation. Laser trimming requires that only a very thin insulating layer cover the fuse because the laser must be able to penetrate the layer and melt the fuse. The portion of the fuse and thin insulating layer over the fuse which is melted away or blown, must not deposit on or interfere with nearby devices. The '041 patent describes an opening over the fuse which is formed of silicon nitride, silicon oxide, spin-on glass and borophosphosilicate glass (BPSG).
The Yoo et al. patent, '041, also describes an insulating layer formed on a semiconductor substrate. A fuse is formed on the insulating layer. Another insulating layer is formed over the first insulating layer and the fuse. A window opening over the fuse is formed at least partially through the second insulating layer. The window exposes either the entire fuse or a portion of the second insulating layer over the fuse. A protective passivation layer is formed on top of the insulating layer. The protective passivation layer has a greater than 50% transmittance of laser irradiation and is formed of silicon nitride.
The Lee et al. patent, U.S. Pat. No. 5,608,257 ('257), which issued Mar. 4, 1997, describes a fuse structure with a melt-away elongated metal fuse link that connects two segments of an interconnection line. The fuse also includes fins integral and coplanar to the fuse link and transversely extending from the fuse link for absorbing energy emitted by the laser beam. The fuse additionally includes a reflecting plate positioned underneath the fuse link for reflecting energy provided by the laser beam back into the fuse link. The fins and reflecting plate reduce the energy emitted by the laser beam required to blow the fuse structure.
The Zagar et al. patent, U.S. Pat. No. 5,677,884 ('884) which issued Oct. 14, 1997, describes an integrated circuit that includes an enable circuit used for enabling one of a collection of redundant elements, and a program circuit for selectively programming the enabled redundant element. The programmed redundant element may be used to replace a defective primary element. The integrated circuit also includes a nonvolatile disable circuit for disabling the enabled redundant elements.
The Billig et al. patent, U.S. Pat. No. 5,025,300 ('300) which issued Jun. 18, 1991, describes a conductive fusible link that is blown by laser energy. A dielectric material covering the fuse is etched away to expose the fuse. A protective dielectric layer is formed on the fuse to a controlled thickness less than that of the interlevel dielectric.